Stabilized lubricants



nited States t 3,115,463 STABILIZED LUBRICANTS Harold D. Orlofit', Gal; Park, and Gordon G. Knapp, Royal Oak, Mich, assignors to Ethyl Corporation, New

York, N.Y., a corporation of Virginia No Drawing. Filed Mar. 18, 1%.), Ser. No. 266,075 14 Claims. ((11. 252-495) This invention relates to improved lubricants. More particularly, it relates to lubricants possessing enhanced resistance against oxidative deterioration.

Industrial and engine lubricantse.g., mineral oils and synthetic diester oilsundergo oxidative deterioration in service, particularly at elevated temperatures. Resulting from this deterioration are the formation of gums and tures, agitated with air, and contain in suspension iron and other metal oxides which catalyze the decomposition of the oil.

An object of this invention is to provide lubricants characterized by enhanced resistance to oxidative deterioration, particularly at elevated temperatures. Another object of this invention is to provide additives which effectively enhance the resistance of engine and industrial oils against oxidative deterioration, especially under the foregoing drastic conditions. A further object is to provide synergistic combinations of additives which very effectively inhibit the oxidative deterioration of lubricating oil. Other important objects will be apparent as the description proceeds.

These and other objects are accomplished by this invention which consists of providing lubricating oil normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of from about 0.001 to about 0.2--and preferably 0.03 to 0.1percent by weight based on the oil of phosphorus as a dialkyl hydrogen phosphite in which the alkyl groups each contain up to about 12 carbon atoms, and from about 0.01 to about 3preferably 0.05 to 1percent by weight based on the oil of a substituted phenolic compound having the formula:

wherein R R and R are hydrogen or alkyl groups containing up to 2 carbon atoms, from 1 to 2 of R R and R being hydrogen; or the formula:

1 s l R1 R1 therein of the specified combination The magnitude "ice of the synergism is very large. For example, it has been found by actual tests that lubricants containing the above synergistic additives have their useful lives prolonged by over 250 times. This is especially striking since none 5 of the above additives when used alone even begins to approximate such effectiveness. In fact, the above phenolic compounds are notoriously weak antioxidants. Many of them are starting materials in chemical processes-cg, alkylationwhich are designed to convert these impotent compounds into acceptable antioxidants.

Yet in the present invention these normally insipid phenolic compounds take on new and unexpectedly great vigor, and even surpass by a very substantial margin the capabilities of highly-publicized alkylated phenol antioxidants.

Economic as well as technical advantages characterize this invention. Plentiful, cheap phenolic compounds normally of little or no value as antioxidants are transformed by this invention into highly potent additive complementsfar more potent than many present-day, commercially-used antioxidants which are high cost, fine chemicals.

The unexpected and disproportionately increased antioxidant effeots of this invention show up under all normal conditions encountered in the manufacture, shipment, storage and use of lubricants. It is especially significant that the synergism is very pronounced under high temperature oxidizing conditions since there is a real need for lubricants that are stable under these conditions. Moreover, these synergistic effects are undaunted by the presence in the oil of deleterious oxidation catalysts, such as iron oxide, which at high temperatures normally promote catastrophic deterioration of lubricants.

The invention is particularly applicable to lubricants which normally tend to deteriorate at elevated temperatures below the cracking temperature of the oil. Hence, the present synergistic additive combinations are very effective in mineral oils and synthetic diester oils. Typical of the latter are diester oils of the type described in industrial and Engineering Chemistry, 39, 484-91 (1947). In general, these diester lubricants have molecular weights from about to less than -100 F. They include 'Qoxalates, m-alonates, succinates, glutarates, adipates, pimlelates, suberates, azelates, sebacates, etc., especially the alkyl and cycloalkyl esters. Esters formed from polyol-s (pentaerytheritol, rtrimethylol propane, etc.) and aliphatic monocarboxylic acids (e.g., fatty acids) are also greatly improved in stability by these additive combinations. The dialkyl hydrogen phosphites used in this invention are those in which the alkylgroups each contain up to about 12 carbon atoms. Typical of these are dimethyl hydrogen phosphite, diisopropyl hydrogen phosphite, disec-butyl hydrogen phosphite, bis-(2-ethylhexyl) hydrogen phosphite, didodecyl hydrogen phosphite, ethyl isoheptyl fhydrogen phosphite, and the like. On the basis of costelfectiveness and ease of manufacture, those made from primary or second alcohols of 1 to 8 carbons are preferred. When used alone, all of these phosphites are reasonably effective antioxidants under mildly oxidizing conditions.

However, their effectiveness falls off sharply at elevated temperatures, especially in the presence of iron oxides. It is thus all-the-rnore remarkable that when these materials are combined with the normally insipid phenolic compounds described herein, such powerful synergistic elfects are achieved even under these drastic conditions.

The phenolic compounds used in this invention are certain rnonoor di-substituted phenols. The structure of these compounds is very important. The very valuable 7 synergistic effects of this invention definitely appear to be tied in, in some unexplainable manner, with the configuration and relative positions of the substituents. This is borne out by tests showing that slight departures from the above specific structural criteria of the phenolic ingredient result not only in loss of synergistic effect, but in a very sharp drop in over-all effectiveness even when such other phenolic compounds are used with dialkyl hydrogen phosphites. It is especially noteworthy that decidedly inferior results have been achieved by combining 4-methyl-2,6-di-tert-butyl phenol with dialkyl hydrogen phosphites. Why this should be the case is a question which presently cannot be answered especially since 4- methyl-2,6-di-tert-butyl phenol is a standard, commercially-used and generally effective antioxidant. Thus, coupled with the complete unexpectedness of the synergistic effects of this invention is the further unexpected contribution of the chemical structure of the phenolic ingredient.

Examples of the phenolic compounds of the lubricants of this invention are o-cresol; p-cresol; mixed oand pcresols; 2-ethyl phenol; 2,6-dimethyl phenol; 2-ethyl-4- methyl phenol; 2,6-diethyl phenol; 4,4-methylenebis(2- ethylphenol); 4,4'-propylidenebis phenol; p,p'-(1-methylbutylidene)diphenol; 4,4'-hexadecylidenedi-o-cersol; p,p- (1 ethylpropylidene)diphenol; p,p' {1-[2-propyl-1) (1- propylbutyl) jpentylidenel diphenol; 4,4 propylidenebis- (2-ethylphenol); and the like. Preferred because of their greater synergistic effectiveness are 2,4-dimethyl phenol; 2,6-dimethyl phenol; and 4,4-isopropylidenebis phenol.

Synergistic effects are exhibited when the above combinations of additives are present in the lubricant in the concentration ranges described above. However, it has been further found that the greatest amount of synergism occurs when the dialkyl hydrogen phosphite and phenolic compound are present in certain relative proportions within the range of the foregoing concentrations. Thus, a particularly preferred embodiment of this invention consists in providing lubricant compositions as described above further characterized in that the dialkyl hydrogen phosphite and phenolic compound are present in relative proportions such that there are from about 0.05 to about moles of the phosphite per mole of the phenolic compound.

In formulating the lubricants, the above synergistic additives are blended in appropriate quantity with the oil, the phosphite and the phenolic compound being used either as a preformed blend or mixture, or by adding them sepa- Y rately in either order.

The following examples illustrate various specific embodiments of this invention. Parts and percentages are by weight. The physical characteristics of the illustrative oils used in Examples 1 through 9 are shown in Table I.

TABLE I.PROPERTIES OF REPRESENTATIVE PETROLEUM HYDROCARBON OILS Oil A B C D E F Gravity at 60 .API 30. 3 30. 5 28. 8 31.1 20. 5 31.0 Viscosity, Saybolt:

Seconds at 100 F 178. 8 373. 8 309.8 169.0 249. 4 335. 4

Seconds at 210 F 51.0 58. 4 63.8 51. 5 45. 7 68. 4 Viscosity Index 154. 2 107.4 141. 9 157.8 35. 8 144. 4 Pour Point 30 +10 15 0 Flash Point 410 465 365 385 Sulfur, percent 0.2 0.3 0.3 0.3 0.3 0. 1

Example 1 To 100,000 parts of oil A is added with stirring 5.3 parts of (0.001 percent of phosphorus as) diisopropyl hydrogen phosphite. To this oil is then added 1000 parts (1 percent) of o-cresol. The resultant oil possesses enhanced resistance to oxidative deterioration.

Example 2 To 100,000 parts of oil B are added 1250 parts of (0.2 percent of phosphorus as) dibutyl hydrogen phosphite and 50 parts (0.05 percent) of 4-ethyl phenol. After mixing, the resulting oil possesses enhanced oxidation resistance.

4 Example 3 With 100,000 parts of oil C are blended 490 parts of (0.05 percent of phosphorus as) dioctyl hydrogen phosphite and 10 parts (0.001 percent) of 2-ethyl-6-methyl phenol. The resulting oil is found to possess very good resistance to oxidative deterioration.

Example 4 To 100,000 parts of oil D are added 1160 parts of (0.1 percent of phosphorus as) di-(decyl) hydrogen phosphite and 500 parts (0.5 percent) of 4,4'-isopropylidenebis phenol. After agitation, the homogeneous lubricant possesses especially good resistance against oxidative deterioration.

To 100,000 parts of oil F are added 540 parts of (0.15 percent of phosphorus as) dimethyl hydrogen phos phite and 3000 parts (3 percent) of 2,6-dimethyl phenol. After mixing, the resultant oil is greatly resistant to oxidative deterioration.

Example 7 With 100,000 parts of oil A are blended 93 parts of (0.08 percent of phosphorus as) bis-'(2-d0decyl) hydrogen phosphite and 20 parts (0.02 percent) of p,p-(1-ethylpropylidene) diphenol. Substantially increased is the resistance of this oil against oxidative deterioration.

Example 8 To 100,000 parts of oil B are added 16.8 parts of (0.002 percent of phosphorus as) butyl-undecyl hydrogen phosphite and 500 parts (0.5 percent) of an essentially equimolar mixture of oand p-cresols. The finished oil is highly resistant to oxidative deterioration.

Example 9 To 100,000 parts of oil C are added 428 parts of (0.06 percent of phosphorus as) diamyl hydrogen phosphite (mixed isomers, prepared from commercially available Oxo amyl alcohols) and parts (0.08 percent) of 4,4- hexadecylidenedi-o-cresol. The mixture is agitated and found to possess good resistance against oxidative deterioration.

Example 10 To 100,000 parts of a commercially-available pentaerythritol ester having a viscosity at F. of 22.4 centistokes and known in the trade as Hercoflex 600 are added 726 parts of (0.09 percent of phosphorus as) bis- (4-methyl-2-pentyl) hydrogen phosphite and 10 parts (0.01 percent) of Z-ethyl phenol. The resultant finished oil possesses much improved resistance against oxidative deterioration.

Example 11 With 100,000 parts of di-(sec-amyl) sebacate having a viscosity at 210 F. of 33.8 Saybolt Universal seconds (SUS), a viscosity index of 133 and a molecular weight of 342.5 are blended 312 parts of (0.05 percent of phosphorus as) di-sec-butyl hydrogen phosphite and 300 parts (0.3 percent) of 4,4'-propylidenebis(Z-ethylphenol). The resistance to oxidative deterioration of the resultant oil is substantially improved.

7 Example 12 To 100,000 parts of di-(2-ethylhexyl) sebacate having a viscosity at 210 F. of 37.3 SUS, a viscosity index of 152 and a molecular Weight of 426.7 are added 180 parts of (0.05 percent of phosphorus as) dimethyl hydrogen phosphite and 546 parts of (0.05 percent of phosphorus as) bis-(2,6-dimethylheptyl) hydrogen phosphite. Next is added 800 parts (0.8 percent) of 2,4-dimethy1 phenol. The resultant homogeneous lubricant has very good resistance against oxidative deterioration.

Example 13 To 100,000 parts of di-(2-ethylhexyl) adipate having a viscosity at 210 F. of 34.2 SUS, a viscosity index of 121 and a molecular weight of 370.6 are added 1120 parts of (0.18 percent of phosphorus as) di-tert-butyl hydrogen phosphite and 200 parts of (2 percent) 2-ethyl-4-methyl phenol. After mixing, the resultant diester lubricant is very resistant to oxidative deterioration.

Example 14 With 100,000 parts of di-(sec-amyl) sebacate having a viscosity at 210 F. of 33.8 SUS, a viscosity index of 133 and a molecular weight of 342.5 are blended 8.9 parts of (0.002 percent of phosphorus as) bis-(2-nonyl) hydrogen phosphite, 500 parts (0.5 percent) of 2,6-diethyl phenol and 200 parts (0.2 percent) of 2-ethyl phenol. The mixture is agitated and the resultant homogeneous lubricant has very high resistance to oxidative deterioration.

Exam ple 15 To 100,000 parts of dioctyl seoacate having a viscosity at 210 F. of 36.7 SUS, a viscosity index of 159 and a molecular weight of 426.7 are blended 106 parts of (0.02 percent of phosphorus as) dipropyl hydrogen phosphite and 40 parts (0.04 percent) of 4,4'-methylenebis phenol. I he mixture is agitated to insure homogeneity. The finished oil is greatly resistant to oxidative deterioration.

In all of the foregoing illustrative examples, the increased resistance to oxidative deterioration results from a substantial synergistic co-action of the additive combinations.

To illustrate the important benefits of this invention a standard oil oxidation test was used. The equipment and test procedure as described by Kroger et al, Erdol and Kohle, 2, 389 (1949), served as the basis of the following tests. The equipment and procedure were slightly modified in order to make the oxidizing conditions even more strenuous. In this manner, the test lubricants were subjected to exceedingly severe oxidizing conditions in order to conclusively establish the effectiveness of the additives under very adverse conditions. Furthermore, the modifications were found to provide results which correlated extremely well with test results of other standard procedures, including actual engine tests.

The equipment consists of a reaction cell connected with an open end manometer whereby the total uptake of oxygen by the oil can be obtained by noting the drop in mercury in the manometer. Thus, the test oil sample is placed in the reaction cell which is flushed with oxygen and the temperature raised to 300 F. and held there until the substrate oil undergoes catastrophic oxidation as shown by the rapid intake of oxygen. In all cases, the substrate oil was deliberately contaminated with iron hexoate as an oxidation promoter (0.05 weight percent of iron as Fe O per 100 grams of oil). By so doing, a very close simulation of stringent oxidizing conditions prevailing with many uses of lubricants was achieved.

In the following tests, the oxidation stability of the test lubricant was determined by measuring its induction period, that is, the time required for catastrophic deterioration to occur under the foregoing conditions. Hence, the longer the induction time, the more stable was the lubricant.

In one series of tests, the base lubricant was a commercially-available, additive-free mineral oil having a viscosity of 87.1 SUS at 100 F. and a viscosity index of 106.5. The additives tested singly and in combination were p-cresol and dimethyl hydrogen phosphite, 0.14 per cent by weight of the former and 0.037 percent by weight of phosphorus as the latter being used. The data and synergistic results are shown in Table II.

TABLE II.EFFECT OF ADDITIVES ON OIL In another example, the presence in the base oil of 0.12 percent by weight of o-cresol and 0.035 percent by weight of phosphorus as dimethyl hydrogen phosphite gave an induction time of 221 minutes. In contrast to this, neither of the additives when used alone at these concentrations even caused the induction time to be lengthened by 20 minutes.

In still another example, the combination of 2,6-dimethylphenol (0.15 percent by weight) and dimethyl hydrogen phosphite (0.038 percent by weight of phosphorus) gave an induction time of 275 minutes. The Very large synergistic effect in clear from the fact that separately, 0.15 percent by weight of 2,6-dimethyl phenol and 0.038 percent by weight of phosphorus as dimethyl hydrogen phosphite give very slight fractions of this value.

The presence in the mineral oil of 0.15 percent by weight of 2,4-dimethyl phenol and 0.031 percent by weight of phosphorus as dimethyl hydrogen phosphite likewise gave an unpredictably-lengthened induction time. The actual value was 347 minutes as compared with the exceedingly small times provided by each of the additives when used separately.

In another series of tests the additives were 4,4'-isopropylidenebis phenol (0.28 percent by weight) and dimethyl hydrogen phosphite (0.046 percent by weight of phosphorus). Table III shows the synergistic data.

Of very great magnitude are the induction times of the lubricants of this invention. This becomes even clearer from the fact that 0.41 percent by weight of 2,2'-meth ylenebis(4-methyl-6-tert-butylphenol), a potent commercially-available antioxidant, only gave an induction time of 44 minutes under the above test conditions.

The great importance of the structure of the phenolic compounds used in this invention is clearly shown by the following comparative tests. Individual blends using the above base oil were made up as follows: Phenol (0.11 percent) plus 0.04 percent of phosphorus; meta-cresol (0.12 percent) plus 0.03 percent of phosphorus; 2,4,6-tri-tertbutyl phenol (0.32 percent) plus 0.03 percent of phosphorus; and 4-rnethyl-2,6-di-tert-butyl phenol (0.23 percent) plus 0.04 percent of phosphorus; in each case the phosphorus being present as dimethyl hydrogen phosphite. With these blends, the induction times were all very small. The phenol-phosphite combination lasted 19 minutes, the meta-cresol combination survived for 22 minutes, the 2,4,6-tri-tert-butyl phenol combination held up for 15 minutes, and the 4-methyl-2,6-di-tert-butyl phenol mixture managed to reach 118 minutes. Obviously, these values are all very small as compared with the excellent results characterizing this invention.

The benefits of this invention are still further demon- 7 strated by carrying out engine tests, such as the CRC L38 test as described in Development of Research Technique for Study of Oxidation Characteristics of Crankcase Oils in the CLR Oil Test Engine, published by the Coordinating Research Council, New York, March 1957. This test very efi'ectively evaluates the oxidation and copper-lead bearing corrosion characteristics of engine crankcase oils. The technique involves operating the CLR oil test engine under constant speed, air/ fuel ratio, and fuel-flow conditions for a total of 40 hours, subsequent to a break-in period of 4.5 hours. Prior to each test the engine is cleaned, pertinent measurements of engine parts are taken and a complete set of new piston rings and. new copperlead connecting-rod test bearing inserts are installed. The chief operating conditions are:

Speed 3150: r.p.rn. Fuel fiow 4.5 to 5.0 lb./hr. Air/fuel ratio 14.0-30.5. Intake-air temperature 80 F., min. Jacket-outlet coolant temperature 200i2 F. Spark advance i-1 BTDC. Oil pressure :2 p.s.i. Crankcase vacuum 2:05 in. water. Exhaust back pressure 0 to 1 in. mercury.

Performance of the oil is judged by visual examination of the engine for deposits, by the weight loss of the test bearing and by comparison of inspection data on used oil samples with the inspection data on the new oil. When a typical additive-free, lubricating oil, such as any of those described in Table I, is subjected to this test, it is found that there has been a substantial weight loss of the test bearings and a considerable amount of deterioration of the oil. However, repetition of the test with the sole variable being that the oil is treated wtih an appropiate concentration of mixtures of dialkyl hydrogen phosphites and phenolic compounds of this invention shows that on completion of the test, there has been a substantial reduction in the weight loss of the test bearings and that the used oil has been virtually unaffected.

Preparation of the phosphites is described in Industrial and Engineering Chemistry, 49, 1871 (1957). Many of the phenolic compounds are commercially-available, the cresols, xylenols, etc., being recovered (from coal-tar. The bisphenols are made by known methods of condensing phenol with the appropriate ketone or aldehyde under acid-catalyst conditions. Preparation of ortho-ethyl phenols is described in co-pending application, Serial No. 426,556, filed April 29, 1954, now Patent No. 2,831,898.

The additives of this invention very eflectively stabilize such lubricating and industrial oils as crankcase lubricating oils, transformer oils, turbine oils, transmission fluids, cutting oils, gear oils, industrial oils, mineral white oils, glass annealing oils, oils thickened with soaps and inorganic thickening agents (grease), and, in general, engine and industrial oils that are normally susceptible to deterioration in the presence of air, particularly at elevated temperatures and most panticularly in the presence of iron oxide.

Aflorded by this invention is greatly enhanced oxidation resistance when the present additive combinations are used in such synthetic ester oils as diethyl oxalate; di-sec-butyl malonate; di-(2-hexyl) succinate; di-(isoheptyl) pimelate; di-(3-decyl) suberate; di-sec-amyl glutarate; di-(isobutyl) glutarate; di-(2-ethylbutyl) glutarate; di(2-ethylhexyl) glutarate; di-sec-amyl adipate; di- (4-propylcyclohexyl) adipate; di-(3-methyl-butyl) adipate; diethyl adipate; di-2-ethylhexyl adipate; di-secamyl vazelate; di-(isobutyl) azelate; di-(2-ethylbutyl) azelate; di-(Z-ethylhexyl) azelate; di-sec-amyl sebacate; di-sec-butyl sebacate; di-(Z-et'hylhexyl) sebacate; bis-(lmethyl-l-cyclohexyl) sebacate; the glutarates, ad-ipates, azelates and sebacates of branched chain secondary alcohols, such as undecanol, tetradeoanol; etc.; the butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, and

dodecyl esters of polyols, such as pentaerythritol, trimethylol propane, rtrimethylol ethane, etc.; and, in general, dicsters of the type described in the literature as useful for synthetic lubricant purposes.

In the compositions of this invention eifective use can be made of other additives which are known in the art, such as other inhibitors, detengent-dispersants, pour point depressants, viscosity index improvers, anti-foam agents, rust inhibitors, oiliness or film strength agents, dyes, and the like. Of the inhibitors which can be eifectively used with the present additive combinations are sulfurized sperm oil, sulfurized terpenes, sulfurized paralfin Wax 0le fins, aromatic sulfides, alkyl phenol sulfides, lecithin, neutralized dithiophosphates, phosphorus pentasulfide-terpene reaction products, diphenylamine, phenylnaphthyl amine, and the like. Typical of the detergent additives that can be used in the compositions of this invention are metallic soaps of high molecular weight acids, such as aluminum naphthenates, calcium phenyl stea-rates, calcium alkyl salicylates, alkaline earth metal petroleum sulfonates, alkaline earth metal alkyl phenol sulfides (barium amyl phenol sulfide, calcium octyl phenol disulfide, etc.), metal salts of wax-substituted phenol derivatives, and the like. Of the viscosity index improvers and pour point depressants, effective use can be made of polymers of the esters of methacrylic acids and higher fatty alcohols and the corresponding polymeric esters of acrylic acid and higher fatty alcohols. These and other additives which can be employed in the compositions of this invention will now be well known to those skilled in the art.

The following tests demonstrate the synergism obtained in synthetic lubricating oils using particular phosphite-phenol combination antioxidants of this invention.

The test procedure and equipment was the same as that described in the tests set forth following Example 15. The oil employed was a di-(Z-ethyl-hexyl) sebacate having a Saybolt viscosity at F. of 68.3, a Saybolt viscosity at 210 F. of 37.3 and a viscosity index of 150.

The results of these tests are shown in Table IV.

TABLE IV Cone. of phosphite (wgt. percent phosphorus) Cone. of

phenolic (wgt. percent) Induction Additivc me (minutes) These data show that the phosphite used had practical- 1y no eifect on the induction period of the oil and that using approximately one-half of the concentration of each of the phosphite and para-cresol tested separately, the effectiveness was practically double that obtained with para-cresol alone at twice the para-cresol concentration.

The above tests demonstrate that a definite synergism is exhibited by the phenol and phosphite combination of this invention.

This application is a continuation-in-part of application Serial No. 713,294, filed February 5, -8, and now abandoned, entitled Stabilized Lubricants.

What is claimed is:

1. A lubricating oil selected from the group consisting of petroleum hydrocarbon oils and synthetic diester oils normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of from about 0.001 to about 0.2 percent by weight based on the oil of phosphorus as'a dialkyl hydrogen phosphite in which the alkyl groups each contain up to about 12 carbon atoms and from about 0.01 to about 3 percent by weight based on the oil of a substituted phenolic compound selected from the class consisting of mononuclear phenols of the formula wherein R R and R are selected from the group consisting of hydrogen and alkyl groups containing up to 2 carbon atoms, from 1 to 2 of R R and R being hydrogen; and bisphenols of the formula wherein R is selected from the group consisting of hydrogen and alkyl groups containnig up to 2 carbon atoms, and R and R are selected from the group consisting of hydrogen and alkyl groups, the total number of carbon atoms in R and R being up to 15.

2. A lubricating oil selected from the group consisting of mineral oils and synthetic diester oils normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of from about 0.001 to about 0.2 percent by Weight based on the oil of phosphorus as a dialkyl hydrogen phosphite in which the alkyl groups each contain up to about 12 carbon atoms and from about 0.01 to about 3 percent by weight, based on the oil, of a substituted phenolic compound selected from the class consisting of mononuclear phenols of the formula wherein R R and R are selected from the group consisting of hydrogen and alkyl groups containing up to two carbon atoms, from one to two of R R and R being hydrogen; and bisphenols of the formula wherein R is selected from the group consisting of hydrogen and alkyl groups containing up to two carbon atoms, and R and R are selected from the group consisting of hydrogen and alkyl groups, the total number of carbon atoms in R and R being up to 15, said dialkyl phosphite and alkylphenol being present in proportions such that there are from about 0.05 to about 5 molecular equivalents of the phosphite per equivalent of the phenol.

3. A lubricating oil selected from the group consisting of mineral oils and synthetic diester oils normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of from about 0.001 to about 0.2 percent by weight; based on the oil, of phosphorus as dimethyl hydrogen phosphite and from about 0.01 to about 3 percent by weight, based on the oil, of 2,4-dimethylphenol.

4. Petroleum hydrocarbon lubricating oil normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of from about 0.001 to about 0.2 percent by weight, based on the oil, of phosphorus as dimethyl hydrogen phosphite and from about 0.01 to about 3 percent by weight, based on the oil, of 2,6-dimethylphenol.

5. Petroleum hydrocarbon lubricating oil normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of from about 0.001 to about 0.2 percent by weight, based on the oil, of phosphorus as dimethyl hydrogen phosphite and from about 0.01 to about 3 percent by Weight, based on the oil, of 4,4-isopropylidenebisphenol.

6. Petroleum hydrocarbon lubricating oil normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of from about 0.01 to about 0.2 percent by weight based on the oil of phosphorus as dimethyl hydrogen phosphite and from 0.01 to about 3 percent by weight based on the oil of a substituted phenolic compound having the formula H O I wherein R R and R are selected from the group consisting of hydrogen and alkyl groups containing up to two carbon atoms, from one to two of R R and R being hydrogen.

7. Petroleum hydrocarbon lubricating oil normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of .from about 0.001 to about 0.2 percent by weight, based on the oil, of phosphorus as dimethyl hydrogen phosphite and from about 0.01 to about 3 percent by weight, based on the oil, of 2,4-dimethylphenol.

8. Petroleum hydrocarbon lubricating oil normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of from about 0.001 to about 0.02 percent by weight, based on the oil, of phosphorus as dimethyl hydrogen phosphite and from about 0.01 to about 3 percent by weight, based on the oil, of p-cresol.

9. Petroleum hydrocarbon lubricating oil normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of from about 0.001 to about 0.2 percent by weight, based on the oil, of phosphorus as dimethyl hydrogen phosphite and from about 0.01 to about 3 percent by weight, based on the oil, of o-cresol.

10. Synthetic diester lubricating oil normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of from about 0.01 to about 0.2 percent by weight, based on the oil, of di-sec-butyl hydrogen phosphite and from about 0.01 to about 3 percent by weight, based on the oil, of a substituted phenolic compound selected from the class consisting of mononuclear phenols of the formula wherein R R and R are selected from the group consisting of hydrogen and alkyl groups containing up to two carbon atoms, from one to two of R R and R being hydrogen.

11. Synthetic diester lubricating oil normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of (from about 0.01 to about 0.2 percent by weight, based on the oil, of di-sec-butyl hydrogen phosphite and from about 0.01 to about 3 percent by weight, based on the oil, of p-cresol.

12. A di-(2-ethyl-hexyl) sebacate synthetic diester lubricating oil normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of from about 0.01 to about 0.2 percent by weight, based on the oil, of di-sec-butyl hydrogen phosphite and from about 0.01 to about 3 percent by Weight, based on the oil, of p-cresol.

13. A lubricating oil selected from the group consisting of mineral oils and synthetic diester oils normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of from about 0.001 to about 0.2 percent by weight based on the oil of phosphorus as a dialkyl hydrogen phosph'ite selected from the group consisting of dimethyl hydrogen phosphite and di-sec-butyl hydrogen phosphite and from 0.01 to about 3 percent by weight based on the oil of a substituted phenolic compound selected from o-cresol, p-cresol, 2,4 dimethylphenol, 2,6 dimethylphenol and 4,4-isopropylidenebisphenol.

14. Mineral oil normally susceptible to oxidative deterioration containing a small antioxidant quantity of a synergistic mixture consisting of from about 0.001 to about 0.2 percent by weight based on the oil of phosphorus as a dialkyl hydrogen phosphite selected from the group consisting of dimethyl hydrogen phosphite and di-sec-butyl hydrogen phosphite and from 0.01 to about 3 percent by weight based on the oil of a substituted phenolic compound selected from o-cresol, p-cresol, 2,4 dimethylphenol, 2,6-dimethylphenol and 4,4'-isopropylidenebisphenol.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 115,463 December 24, 1963 Harold D, 0r1off et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, lines 51 to 57, the formula should appear as shown below instead of'asin thepatentr I I Y column 2, line 47, for "pentaerytheritol" read pentaery thritol column 3, TABLE 1, second column, 'line 3 thereof, for "51.0" read 52.0 column 4, line 4, for "0.001" read 001 column 10, line 37, for "0.02" read 0,2

Signed and sealed this 14th day of July 1964.

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents ESTON GU JOHNSON Attesting Officer UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 115,463 December 24, 1963 Harold D. Orloff et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, lines 51 to 57, the formula should appear as shown below instead ofasin the patents" I T column 2, line 47, for "pentaerytheritol" read pentaerythritol column 3, TABLE I, second column, l1ne 3 thereof for "51.0" read 52.0 column 4, line 4, for "0.001" read 0.01 column 10, line 37, for "0.02" read 0.2

Signed and sealed this 14th day of July 1964.

(SEAL) Attest:

ESTON G.. JOHNSON EDWARD J. BRENNER Attesting 0fficer Commissioner of Patents 

1. A LUBRICATING OIL SELECTED FROM THE GROUP CONSISTING OF PETROLEUM HYDROCARBON OILS AND SYTHETIC DIESTER OILS NORMALLY SUSCEPTIBLE TO OXIDATIVE DETERIORATION CONTAINING A SMALL ANTIOXIDANT QUANTITY OF A SYNERGISTIC MIXTURE CONSISTING OF FROM ABOUT 0.001 TO ABOUT 0.2 PERCENT BY WEIGHT BASED ON THE OIL OF PHOSPHORUS AS A DIALKYL HYDROGEN PHOSPHITE IN WHICH THE ALKYL GROUPS EACH CONTAIN UP TO ABOUT 12 CARBON ATOMS AND FROM ABOUT 0.01 TO ABOUT 3 PERCENT BY WEIGHT BASED ON THE OIL OF A SUBSTITUTED PHENOLIC COMPOUND SELECTED FROM THE CLASS CONSISTING OF MONONUCLEAR PHENOLS OF THE FORMULA 